The present invention relates to a method of controlling the shape of rolled products in the rolling of plate, sheet, strip and the like (hereinafter called plate etc.), more particularly a method of controlling the flatness and cross-sectional contour of rolled objects by controlling the thermal expansion of the rolling roll.
As a method of controlling the shape of rolled objects in the rolling of plate, etc., there is known a method according to which the shape of the gap formed between the upper and the lower working rolls, is made changeable in the direction of the axes of the working rolls by applying bending forces to the working rolls or the back-up rolls thereof by oil pressure or otherwise. Besides said method using external force mechanically applied to rolls, there are available such methods as changing the thermal expansion of the rolls in the direction of the axes of the rolls through changes of the equilibrium between the quantities of heat supplied into and escaping from the rolls either by an adjustment of the quantity of a coolant (water, an emulsion containing oils or fats or the like) sprayed on the surface of the rolls for cooling them, or by changing the distribution of flows of such coolant in the direction of the axes of the rolls.
Also, there is used for such rolling operations as skin pass rolling that use no coolant, a method of changing the shape of rolled objects by the adjustment of thermal expansion of the rolls in the direction of the axes of the rolls through the selection of the capacity of and switching on and off of a great number of gas burners along the surface of the rolls in the direction of the axes of the rolls.
Because rolls play the most important role in keeping the rolling mill rigid, and also for good accuracy of the size of the rolled products, the rolls have a large cross-sectional size. Therefore, in the method of using externally applied mechanical force on the rolls for controlling the shape of rolled objects, in order that such a force be sufficient for bending the rolls it must be very large; otherwise, no effect will be obtained; hence the apparatus for carrying out this method is large. Likewise, it is very difficult to provide an existing rolling mill with a large size apparatus for controlling the shape of rolled objects.
Also, in the methods that have been used as described above, their effect on the shape of rolled objects can be followed theoretically only with difficulty which constitutes a drawback of such methods. Specifically speaking, it is known that the greater the relative quantity of a coolant applied to the middle part of the roll, the smaller the expansion of the middle of the roll; but it is difficult to quantitatively determine the amount of reduced expansion of the middle of the roll. As for the method utilizing the change of rolling load, it may be effective for controlling the shape of rolled objects in only one rolling mill, but it also affects the operation of other rolling mills, requiring special care in using such a method. Besides, by the use of such a method the cross-sectional contour of rolled objects can be controlled only with great difficulty. As for the roll bending apparatus, it is difficult to properly adjust it only by using theoretical considerations, because of the influence of the interlocked factors such as the variance in shape controlling effect according to the change of rolling load, the changing distribution of the rolling load in the direction of the axes of the rolls and also the changing distribution of the rolling force between the working rolls and the back-up rolls in the direction of the axes of the rolls.
Because of the difficulty in applying theoretical considerations for controlling the shape of rolled objects in the rolling of plate, etc. by the conventional methods, actual control has not been carried out theoretically but by using the sight of operators or an output signal from a detector of the outside appearance or the like. For the control of the cross-sectional contour of plate, etc. (the distribution of thickness in the width direction of plate, etc.), a roll bending apparatus having a small controlling effect is not used, but the means for adjusting the supply of roll coolant is used in most cases, which makes it still more difficult to apply theoretical considerations for the determination of the effect of the control. Recently, strips produced in a hot strip mill having a high efficiency have sometimes had concave middle parts as a result of insufficient cooling of the rolls due to unbalance between the cooling of the rolls and production efficiency (Ton/Hr). As the result, it is very difficult to obtain desired shapes of the products made from a coil for cold rolling which coil is formed by rerolling such strip, so that it has become necessary to lower the production rate for compensating for the insufficient cooling. This is a difficulty which occurs in the case where the quantity of heat supplied to the rolls is too great to be offset by heat removal by a roll coolant. How to overcome formation of a thermal crown on the rolls, which causes the production of a middle concave part of the strip, is a difficult problem to solve.
In the case of applying an initial crown to the rolls prior to the operation for solving the above problem, there is used a method of providing an initial crown according to the purpose for which the hot coils are to be used. However, it is easily understood that the initial crown cannot be controlled while the rolls are in operation.